1. Field of the Invention
The invention relates generally to low voltage power conversion, and more particularly to power conversion in digital circuits.
2. Description of the Related Art
Information display screens typically use rows of light emitting devices to display a desired image or compilation of data. The light emitting devices generally require large current sources so that the demand may be met in the event that all the devices must xe2x80x9clight upxe2x80x9d at the same time. Satisfying this need is a problem in portable or handheld devices wherein the amount of current or power available is limited by the size of the current or power generator. To address this problem, small power supplies are typically used in combination with power conversion circuits, or boost regulators in the display device. These boost regulators include several transistors which require a certain level of gate drive voltage for operation. The required voltage level is sometimes higher than that supplied directly by the power source of the device.
Some display drive applications require operating power from a single, low-voltage battery where, for power conversion efficiency, the high voltage supply should be generated directly from the battery voltage. A disadvantage of using a battery with a voltage output of 2.7 volts or less, such as lithium ion batteries, is that the high voltage switching transistors employed in the conversion process cannot be driven by such batteries with a high enough control voltage so as to achieve low on-resistance and acceptable power conversion efficiency. One solution to the problem of inadequate gate drive voltage is to increase the size of the switching transistor of the conversion circuit. However, this increases the losses and the monolithic die size for the conversion circuit. If the switching transistor is a discrete device such as a power MOSFET, separate from the control circuitry, the inadequate gate drive increases the size and cost of this component.
In view of the above, it will be appreciated that an appropriate apparatus and method for increasing the voltage supply level while minimizing losses and maintaining an effective die size would be beneficial to power conversion circuits operating from low voltage power sources.
A device for increasing the voltage and current level of a low voltage input signal and a logic signal. The device comprises a voltage multiplier configured to increase the level of a first voltage input signal so as to define a multiplied voltage signal, a level shifting circuit configured to shift a low voltage level of the logic signal to that of the multiplied voltage signal to produce a shifted, multiplied voltage signal, and a device, connected to a second voltage power source and responsive to the shifted, multiplied voltage signal, and configured to increase the current level of the shifted, multiplied voltage signal. In one embodiment, the device for increasing the current level of the shifted, multiplied voltage signal can be a source follower MOSFET. The device for increasing the voltage and current level may be configured to drive a gate of a power switching transistor. Furthermore, the voltage multiplier of the device may be a diode tripler.
One feature of the invention relates to method of increasing the voltage level and current level of a low voltage logic signal using a first voltage source. The method comprises multiplying a first voltage signal from said first voltage source to produce a multiplied voltage signal, and shifting said low voltage logic signal to the voltage level of said multiplied voltage signal. The method may also further comprise increasing the current of said shifted, multiplied voltage signal, by use of a bootstrap connection to a second voltage source.
Another aspect of the invention concerns a method of providing a gate drive voltage to a power transistor from a low voltage input. The method comprises providing a switching control input and multiplying said low voltage input using said switching control input, a diode tripler, and at least one capacitor. The method further comprises level shifting said multiplied low voltage input using said switching control input, and bootstrapping a high voltage switching power supply to a source follower. The method may further comprise providing a first current to said source follower, and providing a second current from said source follower to said power transistor.